I am trying to upload a list into Marketo. This is a rented list, so I want to skip syncing it with salesforce so I use the "skip new people and updates" choice, which seems to be the only way to do that. The list is roughly 100K people and when the import is complete it says it has skipped ~96K. In addition, the people that were imported seem to have information from SFDC pulling through - so it appears that it is not allowing me to upload "new" people into either salesforce OR marketo. How do I include a list in Marketo that is not synced with SFDC if this is not the appropriate way?

If you import a list into Marketo, the people in the database will have fields updated and those not in the database will sync to Marketo but will not automatically sync to SFDC. You would have to either sync them manually using the "Sync person to SFDC" flow step or change their SFDC campaign membership, etc. This means that those 4k already in your database had data values updated. Those 96k that were not in the database were skipped because you chose to exclude them.

On another note, I would always be cautious of importing rented lists, you run a high risk of importing bad data including bounced email addresses.

New people added to Marketo do not sync over to CRM by default. You actually have to initiate that yourself via a smart campaign (or manually). Typically, these are built as centralized operational programs. The "skip new people" option will skip any person that isn't already in Marketo - it has nothing to do with the sync to CRM. So basically, you have about 96k people on that list that aren't already in Marketo.